At first glance the 25 artifacts displayed in the courtyard of a former convent just off the Tiber River here on Tuesday seemed to have little in common: three first-century B.C. fresco fragments from Pompeii were exhibited alongside fifth- and sixth-century B.C. Etruscan and Attic vases, a 17th-century Venetian cannon, a 12th-century mural fragment depicting Christ and three rare 17th-century books. What they shared was a nefarious past.